Start with safety
Electricity and water together are the most immediate hazard. Stay away from outlets, cords and fixtures in or near standing water, and do not enter a flooded lower level if the panel or wiring may be affected.
If a ceiling is sagging or dripping, keep people out of the room beneath it until it has been examined.
Stop the source if it is safe
Where the water is coming from a supply line, fixture or appliance, closing the local shut-off or the main valve limits how much more water enters the property. Only do this if you can reach the valve safely and dry.
Protect what you can
Move small valuables, documents and electronics out of the affected area. Lift fabric items off wet flooring and place foil or blocks under furniture legs to prevent staining.
Photograph the affected areas before anything is moved or removed. These photographs are often useful later.
Arrange an assessment
Water spreads through materials that look dry from the surface. A professional assessment records how far moisture has travelled and what that means for the flooring, drywall and structure involved.
This guide is general information. Conditions vary between properties, so decisions about your building should follow an assessment of the affected areas.